Semi-empirical modelling of hydraulic conductivity of clayey soils exposed to deionized and saline environments

Theoretical and numerical study of contaminant transport in clayey barriers using a revised numerical model considering the dependency of membrane efficiency and hydraulic conductivity on solute concentration

Solid waste is often buried in landfills isolated with a bentonite-based clay barrier to guarantee the high quality of groundwater. As the efficiency of clay barriers is highly dependent on solute concentration, this study aims to modify membrane efficiency, effective diffusion, and hydraulic conductivity of bentonite-based clayey barriers exposed to saline environments for numerical investigation of solute transport in such barriers. Therefore, the theoretical equations were modified as a function of solute concentration instead of constant values. First, a model was extended for membrane efficiency as a function of void ratio and solute concentration. Second, an apparent tortuosity model was developed as a function of porosity and membrane efficiency to adjust the effective diffusion coefficient. Moreover, a recently developed semi-empirical solute-dependent hydraulic conductivity model was employed, which is dependent on solute concentration, liquid limit, and void ratio of the clayey barrier. Afterward, four approaches for applying these coefficients were defined as either "variable" or "constant" functions in ten numerical scenarios using COMSOL Multiphysics. The results reveal that "variable" membrane efficiency affects the outcomes in lower concentrations, while "variable" hydraulic conductivity is more influential in the domain of higher concentrations. Although all approaches converge to the same ultimate distribution of solute concentration using the Neumann exit boundary condition, the choice of different approaches clearly affects the ultimate state for the Dirichlet exit boundary condition. As the thickness of the barrier increases, the ultimate state is reached later, and choosing the approach to apply coefficients is more influential. Decreasing the hydraulic gradient postpones the solute breakthrough in the barrier, and picking the variable coefficients is more crucial in higher hydraulic gradients.